The invention relates to a light barrier with a transmitter, a reflector and a radiation receiver, in which optical means are provided for beaming the radiation emitted by the transmitter to the reflector and for beaming the radiation reflected by the reflector to the receiver, as well as additional means for polarizing the radiation.
So-called reflection light barriers, in which the radiation is directed from the transmitter to the receiver via a reflector and any decrease in the intensity of the radiation received as a result of an interruption of the path of radiation between the transmitter, reflector and receiver is utilized, by an appropriate analyzing circuit, for putting out an alarm signal are known for example, from German Patent Specification No. 1,934,321 or German Patent Specification No. 2,014,107. The reflector is for the most part in the form of a so-called bull's-eye (cat's eye) with a number of tetrahedral elements which, independently of the inclination, at all times reflect the incident radiation precisely in the direction of incidence, so that the transmitter and receiver can be placed in close proximity and connecting lines between the transmitter and receiver can be dispensed with.
It is also possible, however, to provide several reflectors, between which the radiation is thrown back and forth, so that a laminar protective curtain is created.
In principle, it is possible to use as radiation any desired electromagnetic radiation that can be beamed by optical means, such as light, for example. In order for a light barrier, or rather, its protective beam, not to be readily detectable, it is preferable that the light barrier is made of invisible radiation, and in particular, infrared radiation, which for the purposes of the invention, is included in the concept of light radiation.
In the event that no suitable security precautions have been taken, it is easy to outwit and sabotage such light barriers by irradiating the receiver by means of a substitute transmitter with radiation of the same wavelength. With a view toward remedying this and toward making light barriers more safe against sabotage, attempts have been made to codify the radiation intensity, for example as a pulse train with a specified frequency, or to work with polarized radiation, as for example by placing a polarizing filter in front of the transmitter and the receiver, or through the use of a radiation source emitting polarized radiation. Even such light barriers, however, can easily be sabotaged and rendered ineffectual by sliding an appropriate substitute reflector, such as an analogous bull's-eye, in front of the radiator and receiver, so that the receiver continues to receive unattenuated radiation, while the interval between the substitute reflector and the light barrier reflector is unprotected and can be passed through by an intruder.